The present invention relates to piezoelectric actuators. The invention particularly relates to a polymer-filled and unfilled actuators including a ceramic tube or rectangular, parallel ceramic blade pair, supported by a stiff base, and arrays of such actuators.
U.S. Pat. No. 5,340,510 discloses 1-3 and 2-2 composite actuators made up of a net-shape molded array of very fine, parallel, piezoelectric or electrostrictive ceramic elements in a polymer matrix. The elements of the 1-3 actuator are rod-shaped, while those of the 2-2 actuator are blade-shaped. The composite body is electroded on its planar upper and lower surfaces and poled to produce a composite actuator array. This composite actuator array has been a great improvement over the prior art, but further improvement is desired for low-voltage operation. The operating voltage of such an actuator array is limited by the height of the rods, which is the dimension along which the voltage is applied in operation of the array.
Particularly promising for a low-voltage actuator is a configuration substituting a ceramic tube for each rod-shaped element of U.S. Pat. No. 5,340,510. The advantages of utilizing a tube instead of a solid ceramic rod in such a actuator array are that the tube can be poled and driven by a relatively low voltage applied across the wall thickness of the tube, while a high mechanical displacement can be obtained along the tube height direction. The tube actuator can be made to operate at a very low voltage by suitably reducing the ceramic tube wall thickness, which is a great advantage in many applications. Additionally, the tube height displacement can be increased independently of the applied voltage by increasing the tube height while keeping the wall thickness constant.
Prior art piezoelectric sensors are made from ceramic tubes arranged in a periodic array. However, until the present invention, tubes have not been fabricated in an array attached to a ceramic baseplate, which can be used to facilitate electrical connection to the inner and outer tube walls for operation of the array. In the prior art, individual tubes with attached wiring have been fabricated and mounted one at a time onto a baseplate to form an array. This, however, is clearly a costly process, and it is difficult to ensure that all tubes are electrically connected properly in parallel. Also, the tubes found in the prior art are unfilled and open at both ends, a configuration unsuitable for strain amplification.
Accordingly, it is an object of the present invention to provide a piezoelectric actuator which overcomes the disadvantages of the prior art.
It is another object of the invention to provide a strain-amplified piezoelectric actuator which utilizes a polymer-filled ceramic enclosure or cavity, e.g., a tube or parallel pair of blades, with voltage across the enclosure wall thickness to produce vibration in the wall or walls which is transmitted to a desired medium by way of the polymer.
It is another object of the invention to provide a strain-amplified piezoelectric actuator device having an strain-amplified piezoelectric actuator array including a array of such polymer-filled actuators.
It is another object of the invention to provide a stiff cover plate bonded to the polymer filling the ceramic enclosure for transmitting the enclosure wall vibration to a desired medium.
It is another object of the invention to provide a piezoelectric actuator device having an array of fine ceramic tube-shaped actuators which are unfilled or filled with a soft compressible polymer, and are poled and electroded to operate in a d.sub.31 mode, i.e., with vertical vibration of the tube walls, and including a stiff cover plate bonded to the ceramic walls of the tubes for transmitting the wall vibration to a desired medium.